We plan to study the regulation of enzymes requiring B6 and folic acid, involved in the metabolism of serine and threonine. Special emphasis will be placed on the relationship of these enzymes to neogenesis of labile methyl groups and the relationship of these reactions to homocystinuria. Such a study will help to delineate preferred metabolic pathways and control mechanisms in mammalian systems. We hope to also gain a better understanding of how pyridoxal phosphate and folate requiring enzymes operate from a study of the enzymes: serine-threonine dehydratase, serine hydroxymethylase-threonine aldolase and methylene tetrahydrofolate reductase.